The present invention relates to a storage system which stores files used by computers. More specifically, the invention relates to a storage system capable of saving the resource while exerting a WORM function.
When holding electronic data in a memory device such as a storage, there are problems in that as a retention period of the electronic data becomes longer and an amount of data increases, a demanded capacity of the storage also increases, requiring higher costs. Therefore, more important data is recorded on a high-speed storage with a large transfer bandwidth, and less important data is recorded on a relatively low-speed storage. Such a high-speed storage with the large transfer bandwidth costs high, and such a relatively low-speed storage costs low. Further, based on the date when the data is saved (i.e., the archive date), fresh data (data for which not much time has elapsed since being archived) is recorded on the high-speed storage, and old data (data for which a given duration of time has elapsed since being archived) is recorded on the low-speed storage. This type of data management method is called Data Lifecycle Management (DLCM).
One of known file archive mechanisms which are designed by taking into account long-term file retention in this data lifecycle management is Write Once Read Many (WORM) archive, which manages modification, and erasure of files whose archive period has not expired yet. In order to apply WORM to a memory device, a storage device or a controller has to have a WORM function.
An example of a WORM function for Network Attached Storage (NAS) is a technology proposed to make any erasure, modification or such on a file that is stored under a specific directory impossible by setting the mode bit of the file to “Read Only” through a NAS server (see, for example, the following internet article:    Network Appliance, “NetApp NearStore”, [online],    <URL: http://www.netapp.com/products/nearstore/>).